The present invention relates to an optical transmission system and in particular to an optical transmission system for use in an optical communications system such as a submarine optical communications system or a terrestrial optical communications system.
In submarine optical communications systems there is an increasing demand for transmission capacity due for example, to the ever increasing use of the internet around the world. This can be achieved by increasing the bandwidth of the communications system or by increasing the information content transmitted within the existing system bandwidth. In Dense Wavelength Dependent Multiplexed communications systems (DWDM) having a plurality of transmission channels, channel spacings are approaching the spectral linewidths of individual channels and so in such systems it is not possible simply to increase the volume of information transmitted as there are no available free wavelengths. This problem is exacerbated by the use of return-to-zero (RZ) format for the information transmission and the phase modulation techniques that are widely used which give rise to a relatively broad channel spectrum.
One proposed method of addressing this problem is to use the L-band of the optical frequency spectrum ie transmit information at wavelengths between 1570 nm and 1610 nm. However, this would require the redesigning of optical amplifiers and the consequential replacement of components in existing systems since these were not originally designed for operation in the L-band. This would be expensive and would lead to inevitable disruption of traffic whilst the system was being upgraded.
According to a first aspect of the present invention, an optical transmission system for an optical communications system, comprises: an optical source; an optical modulator for modulating an optical output of the optical source; an optical filter arranged to substantially remove one of the upper and lower sidebands of the modulated optical output of the optical source; and, control means to control at least one of the optical source and the optical filter to ensure that substantially only half of the power of the modulated optical output of the optical source is transmitted, thereby reducing the bandwidth of the optical signals transmitted by the system.
The present invention provides an optical transmission system arranged to provide an optical signal having had either its upper or lower sideband removed by an optical filter. By removing one of the sidebands, the bandwidth of the signal and therefore the required wavelength separation of individual wavelength channels can be substantially reduced so that the number of channels can be increased and/or the power of each channel can be increased.
Preferably, a vestige of either the upper or lower sideband is also transmitted.
Since the wavelength spectrum of the modulated output of the optical source is symmetrical about the carrier wavelength, the information content in the spectrum is duplicated.
Therefore, when one half of the signal is removed, leaving only a single sideband and a vestige of the removed sideband, very little information is lost.
Preferably, the control means is arranged to move the wavelength spectrum of the modulated optical output of the optical source and a transmission profile of the optical filter relative to each other to control the power of the transmitted signal.
More preferably, the control means comprises a feedback loop arranged to detect reflected light from the optical filter and transmitted light from the optical filter and to move the wavelength spectrum of the modulated optical output of the optical source and the transmission profile of the optical filter relative to each other to ensure the detected power of the transmitted light and the detected power of the reflected light remain substantially equal. The use of a feedback loop enables the control of the power of the transmitted and reflected light in the system to be effected automatically.
Preferably, the feedback loop comprises a first branch having an optical sensor arranged to detect the intensity of the reflected light from the optical filter and a second branch having an optical sensor arranged to detect the intensity of the transmitted light from the optical filter. Preferably the feedback loop comprises control means arranged to receive outputs from optical sensors in the first and second branches of the feedback loop, respectively, and vary the wavelength of the laser source and/or the position of the rising edge of the transmission profile of the optical filter accordingly.
Preferably, the optical modulator is a data driven optical modulator such as a Mach Zehnder modulator, for example.
Preferably, the optical filter is an optical fibre grating. Preferably, the optical filter comprises an optical fibre grating having a notch transmission profile, wherein a carrier frequency of the modulated optical output is controlled to substantially coincide with the rising edge of the notch transmission profile, thereby ensuring that substantially half of the modulated optical output of the optical source is transmitted and half is rejected.
In one example, the control unit is arranged to control the temperature of the optical fibre grating, thereby controlling the position of the rising edge of the transmission profile relative to the wavelength spectrum of the modulated optical output of the optical source.
Preferably, a first optical coupler, for example an optical circulator, is included in the transmission system arranged to couple the modulated optical output of the optical source from a transmission path in the transmission system to the optical filter positioned in the first branch of the feedback loop, and couple reflected light from the optical filter back to the transmission path in the transmission system.
More preferably, a second optical coupler is provided arranged to couple light from the first optical coupler to the optical sensor positioned in the second branch of the feedback loop.
According to a second aspect of the present invention, a line terminal endstation for a wavelength division multiplexed communications system, comprises a number of optical transmission systems for generating a number of individual channels, in which at least one of the optical transmission systems is an optical transmission system according to the first aspect of the present invention.
According to a third aspect of the present invention, an optical communications system comprises at least one line terminal endstation according to the second aspect of the present invention.
Preferably, the communications system is a submarine communications system. Preferably the communications system is a terrestrial communications system.